Case Study 4: Cables Enable Reinforcement of the Grid in a Built-Up Area

When the UK's National Grid Company needed to provide extra power into North-West London to meet growing demand, it was not possible to provide the transmission capacity using overhead lines as such lines would require both extremely large towers and a wide right-of-way along a route that was already heavily developed. The alterative was to install underground cable in a tunnel, which would allow the project to run with very little above ground disturbance.

The final design involved a 20 km long tunnel running from Elstree in Herfordshire to St John's Wood in North London at a depth of around 20-30 meters below ground level - although the maximum depth is around 80 meters in one stretch. The tunnel has an internal diameter of 3 meters and contains a single circuit run of 400kV XLPE (cross-linked polyethylene) cable, the cable alone weighing almost 2,500 tonnes. The cables are maintained and inspected via a monorail-mounted inspection system. To future proof the project, the tunnel was built with capacity to hold another cable circuit. The project included seven head house buildings along the cable and two new transformer substations at each end and had a budget of £200M. The project started in March 2000 and was commissioned in September 2005.

In addition to using modern tunnel boring technology to offer almost no disruption to people above the tunnel line, the project also employed advanced monitoring and planning techniques to ensure that there were no collisions between the boring equipment and existing infrastructure.

Areas where care had to be taken included Staples Corner, where road bridges on the M1 have deep foundation pilings and existing utility structures such as the Thames Water Ring Main, a major sewer system and existing electricity cables.

The project forms an important part of the London Connection Project, which is intended to reinforce power transmission into London. Much of the existing 275kV infrastructure is coming to the end of its life and is undersized for projected demand. Progressively overlaying and replacing the 275kV lines with 400kV lines and cables will significantly improve capacity whilst maintaining continuity of supply to the UK capital.

The success of this project has prompted adoption of a similar unobtrusive tunneling approach that will be used to install a second 400KV cable circuit in the UK between Rowdown and Beddington in 2010.